Caractérisation mécanique et microstructurale des joints brasés d'alliages aéronautiques avec du BNI-2 comme métal d'apport

Abstract

The present project aims to evaluate the mechanical properties of nickel based brazed joints in tensile and shear, and to establish their fracture mechanism. The main factors studied are: the overlap length, the braze thickness, the base metal thickness, the ratio of overlap to the thickness of the base métal and as well the presence or not of the eutectic phases in the brazed joint. Their effects on mechanical properties and fracture mechanisms have been investigated. The brazing process bas been carried out according to the AMS 2675 spécification using the AWS BNi-2 (AMS 4777) filer métal. The base metal used was the Inconel 625 (AMS 5599). Tensile tests were carried out according to the ASTM E08-4 specification and results were analyzed based on the AWS C3.2M spécification and some analytical models. The results obtained show that the presence of eutectic phases in the brazed joints drastically decreases their strength. For typical overlap lengths near 4 mm (0.150 in), the brazed joints without eutectics are clearly stronger than those with eutectics. A difference of 50% bas been observed for brazed joints with a larger gap and about 25% for those with a smaller gap. Moreover, with an overlap ratio of 2 and over, the brazed joints without eutectics exhibit about 90% of the pure base metal tensile strength. For all joints tested, it bas been observed that cracks always initiate at the surface of the filet, in the eutectic zone. The propagation however dépends on the overlap and the microstmcture. The presence of a continuons eutectic zone throughout the joint seems to facilitate the crack propagation, while the y-nickel phase free of eutectics seems more ductile and does not show any évidence of crack propagation, even for very short overlap lengths. Therefore, the brazed joints without eutectics exhibit ductile behavior and tend to fail in the base metal, this suggests a good potential for stmcmral applications. A complementary study bas been carried out at the end of this project. The goal was to compare braze mechanical properties to the machined base métal in simple overlap configuration. The resuhs obtained show that in absence of eutectics in the joint and at the fillets, the brazed joints bave similar mechanical properties to that of the base metal. Concretely, the complete absence of eutectics increases braze mechanical strength and ductility respectively to 40 % and 60 % while compared to the pure base metal.

Date	6 May 2010
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Philippe Bocher (Supervisor)